In portable electronic devices such as cell phones, PDA (Personal Digital Assistants) and digital cameras, microprocessors are provided to control the system inside the devices, and the microprocessors perform control by monitoring temperature, voltage of batteries and so on. Therefore, sensors for detecting temperature, voltage of batteries and so on are provided in the devices, and microprocessors in which an A/D conversion circuit which converts analog signals from the sensors in to digital signals is installed are commonly used.                Moreover, as for the A/D conversion circuit installed in a microprocessor or the like, it is preferred that the circuit size is small. As for such A/D conversion circuit, an A/D conversion circuit including a so-called chopper-type comparator which uses CMOS inverters as amplifiers as shown in FIG. 14, for example, is known.        
In such A/D conversion circuit, input signal Vin is sampled to a capacitance Cs by setting a voltage of logical threshold value of the inverter as the reference, by turning on the switch (sampling switch) SS1 in the input side of the analog signal in a state where input/output terminals of the CMOS inverters are short circuited by a sampling clock. Thereafter, the sampling switch SS1 is turned off, and a comparison voltage Vref is applied to the sampling capacitance Cs by turning on the switch SS2 in the input side of comparison voltage and the output changes by each of the inverters operating as an amplifier by blocking the input/output of the CMOS inverters. At this time, the input is amplified by inverters of three stages. Therefore, the output is a power voltage Vcc or a grounding potential GND approximately at the logical level, and judgment result of magnitude relation of Vin and Vref is outputted.